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148 lines
7 KiB
Markdown
148 lines
7 KiB
Markdown
Some notes on how we use git
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============================
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On keeping the commit history clean
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-----------------------------------
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In an ideal world, our git commit history would be a linear progression of
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commits each of which contains a single change building on what came
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before. Here, by way of an arbitrary example, is the top of `git log --graph
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b2dba0607`:
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<img src="img/git/clean.png" alt="clean git graph" width="500px">
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Note how the commit comment explains clearly what is changing and why. Also
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note the *absence* of merge commits, as well as the absence of commits called
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things like (to pick a few culprits):
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[“pep8”](https://github.com/matrix-org/synapse/commit/84691da6c), [“fix broken
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test”](https://github.com/matrix-org/synapse/commit/474810d9d),
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[“oops”](https://github.com/matrix-org/synapse/commit/c9d72e457),
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[“typo”](https://github.com/matrix-org/synapse/commit/836358823), or [“Who's
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the president?”](https://github.com/matrix-org/synapse/commit/707374d5d).
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There are a number of reasons why keeping a clean commit history is a good
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thing:
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* From time to time, after a change lands, it turns out to be necessary to
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revert it, or to backport it to a release branch. Those operations are
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*much* easier when the change is contained in a single commit.
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* Similarly, it's much easier to answer questions like “is the fix for
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`/publicRooms` on the release branch?” if that change consists of a single
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commit.
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* Likewise: “what has changed on this branch in the last week?” is much
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clearer without merges and “pep8” commits everywhere.
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* Sometimes we need to figure out where a bug got introduced, or some
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behaviour changed. One way of doing that is with `git bisect`: pick an
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arbitrary commit between the known good point and the known bad point, and
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see how the code behaves. However, that strategy fails if the commit you
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chose is the middle of someone's epic branch in which they broke the world
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before putting it back together again.
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One counterargument is that it is sometimes useful to see how a PR evolved as
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it went through review cycles. This is true, but that information is always
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available via the GitHub UI (or via the little-known [refs/pull
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namespace](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/checking-out-pull-requests-locally)).
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Of course, in reality, things are more complicated than that. We have release
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branches as well as `develop` and `master`, and we deliberately merge changes
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between them. Bugs often slip through and have to be fixed later. That's all
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fine: this not a cast-iron rule which must be obeyed, but an ideal to aim
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towards.
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Merges, squashes, rebases: wtf?
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-------------------------------
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Ok, so that's what we'd like to achieve. How do we achieve it?
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The TL;DR is: when you come to merge a pull request, you *probably* want to
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“squash and merge”:
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.
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(This applies whether you are merging your own PR, or that of another
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contributor.)
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“Squash and merge”<sup id="a1">[1](#f1)</sup> takes all of the changes in the
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PR, and bundles them into a single commit. GitHub gives you the opportunity to
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edit the commit message before you confirm, and normally you should do so,
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because the default will be useless (again: `* woops typo` is not a useful
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thing to keep in the historical record).
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The main problem with this approach comes when you have a series of pull
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requests which build on top of one another: as soon as you squash-merge the
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first PR, you'll end up with a stack of conflicts to resolve in all of the
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others. In general, it's best to avoid this situation in the first place by
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trying not to have multiple related PRs in flight at the same time. Still,
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sometimes that's not possible and doing a regular merge is the lesser evil.
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Another occasion in which a regular merge makes more sense is a PR where you've
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deliberately created a series of commits each of which makes sense in its own
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right. For example: [a PR which gradually propagates a refactoring operation
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through the codebase](https://github.com/matrix-org/synapse/pull/6837), or [a
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PR which is the culmination of several other
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PRs](https://github.com/matrix-org/synapse/pull/5987). In this case the ability
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to figure out when a particular change/bug was introduced could be very useful.
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Ultimately: **this is not a hard-and-fast-rule**. If in doubt, ask yourself “do
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each of the commits I am about to merge make sense in their own right”, but
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remember that we're just doing our best to balance “keeping the commit history
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clean” with other factors.
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Git branching model
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-------------------
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A [lot](https://nvie.com/posts/a-successful-git-branching-model/)
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[of](http://scottchacon.com/2011/08/31/github-flow.html)
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[words](https://www.endoflineblog.com/gitflow-considered-harmful) have been
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written in the past about git branching models (no really, [a
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lot](https://martinfowler.com/articles/branching-patterns.html)). I tend to
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think the whole thing is overblown. Fundamentally, it's not that
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complicated. Here's how we do it.
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Let's start with a picture:
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It looks complicated, but it's really not. There's one basic rule: *anyone* is
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free to merge from *any* more-stable branch to *any* less-stable branch at
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*any* time<sup id="a2">[2](#f2)</sup>. (The principle behind this is that if a
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change is good enough for the more-stable branch, then it's also good enough go
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put in a less-stable branch.)
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Meanwhile, merging (or squashing, as per the above) from a less-stable to a
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more-stable branch is a deliberate action in which you want to publish a change
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or a set of changes to (some subset of) the world: for example, this happens
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when a PR is landed, or as part of our release process.
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So, what counts as a more- or less-stable branch? A little reflection will show
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that our active branches are ordered thus, from more-stable to less-stable:
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* `master` (tracks our last release).
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* `release-vX.Y` (the branch where we prepare the next release)<sup
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id="a3">[3](#f3)</sup>.
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* PR branches which are targeting the release.
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* `develop` (our "mainline" branch containing our bleeding-edge).
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* regular PR branches.
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The corollary is: if you have a bugfix that needs to land in both
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`release-vX.Y` *and* `develop`, then you should base your PR on
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`release-vX.Y`, get it merged there, and then merge from `release-vX.Y` to
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`develop`. (If a fix lands in `develop` and we later need it in a
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release-branch, we can of course cherry-pick it, but landing it in the release
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branch first helps reduce the chance of annoying conflicts.)
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---
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<b id="f1">[1]</b>: “Squash and merge” is GitHub's term for this
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operation. Given that there is no merge involved, I'm not convinced it's the
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most intuitive name. [^](#a1)
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<b id="f2">[2]</b>: Well, anyone with commit access.[^](#a2)
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<b id="f3">[3]</b>: Very, very occasionally (I think this has happened once in
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the history of Synapse), we've had two releases in flight at once. Obviously,
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`release-v1.2` is more-stable than `release-v1.3`. [^](#a3)
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